The field of this invention is the purification and recovery of diolefins, in particular C.sub.4 - and C.sub.5 - diolefins, such as butadiene and isoprene.
In the manufacture of ethylene and/or propylene by the thermal cracking of naphtha, LPG, gas, oil, or fractions thereof, a hydrocarbon fraction containing conjugated diolefins may be obtained. From this fraction may be recovered a C.sub.4 - fraction comprising 1,3-butadiene, n-butanes, n-butenes, isobutene, vinyl acetylene, ethyl acetylene, 1,2-butadiene, and other C.sub.4 -'s. A C.sub.5 - fraction comprising isoprene, 1,3-pentadiene, cyclopentadiene, paraffinic compounds (e.g. isopentane and n-pentane), olefins (e.g. 2-methyl butene-1 and pentene-1), acetylenes (e.g. pentyne-1 and 1-pentene-4-yne), and other hydrocarbons (e.g. benzene, hexane, cyclopentane, cyclopentane and dicyclopentadiene) may also be recovered.
There are numerous patents on the recovery of 1,3-butadiene from the C.sub.4 - fraction or of isoprene from the C.sub.5 - fraction, many of which employ extractive distillation using any one of a variety of solvents and a variety of chemical unit operations. Some of the U.S. patents pertinent to this subject matter are:
Nos. 2,407,997 and 2,426,705 -- a single extractive distillation and followed by water wash to remove solvent (e.g. acetone) and distillation;
No. 2,437,230 -- separating butadiene from butylenes using substantial water with extraction solvent (e.g. 10-55 mole percent);
No. 2,459,403 -- two extractive distillations plus an azeotropic distillation;
No. 2,971,036 -- single extractive distillation plus cyclopentadiene dimerization and subsequent dimer removal;
No. 3,317,627 -- alkynes removed as a side stream after a single extractive distillation;
Other patents pertinent to this field are Nos. 2,993,841; 3,803,258; 3,795,588; 3,655,806; 3,436,437; 3,201,492; and 2,623,844.
In diolefin purification, removal of acetylenic hydrocarbons is of particular importance. Diolefins such as 1,3-butadiene and isoprene are monomer raw materials for the production of synthetic rubber by catalytic solution polymerization. Acetylenes, such as 2-methyl-1-butene-3-yne, inhibit the desired polymerization reaction in concentrations as small as 300 ppm (parts per million). They react with polymerization catalysts, increasing catalyst consumption. For most solution polymerization work, acetylenes should be limited to 100 to 400 ppm maximum concentration, preferably less than 100 ppm.
Several patents have shown the use of extractive distillation processes involving two or more extractive distillations to remove acetylenes from diolefin-containing streams. U.S. Pat. Nos. 3,510,405 and 3,436,438 describe two-stage extractive distillation processes. The first extractive distillation removes paraffins and monoolefins as distillate or raffinate. The extract is stripped so the solvent can be recycled. The stripper distillate is subjected to a second extractive distillation which removes acetylenes as extract. The second extract is stripped in a second stripper to purify the solvent for recycle. Further purification of the isoprene distillate stream from the second extractive distillation is by ordinary distillation. Distillation steps for removal of CPD (cyclopentadiene) may be added according to U.S. Pat. No. 3,510,405.
U.S. Pat. No. 3,784,626 represents another two-stage extractive distillation process similar to 3,510,405 except that both isoprene and dicyclopentadiene leave the process by distillation column side streams, and the extraction solvent is different. Another process which utilizes a two-step extractive distillation and discharges isoprene as a side stream (but in which the isoprene is taken in the raffinate of the first extractive distillation instead of the extract) is described in U.S. Pat. No. 3,851,010.
A three-stage process (i.e. three extractive distillations) is set forth in U.S. Pat. No. 3,860,496. The first extractive distillation (like those of many of the other patents mentioned) removes paraffins and olefins. The second extractive distillation removes cyclopentadiene (CPD) and acetylenes as a side stream, and the third one removes CPD and acetylenes as extract.
The subject of the present invention is a two-stage extractive distillation process which accomplishes the removal of acetylenes from diolefin-containing streams but which utilizes fewer chemical unit operations and a lower extraction solvent flow than processes such as that represented by U.S. Pat. No. 3,436,438, discussed above.